Epidemiological surveys suggest that one-third of the world's population is infected with Mycobacterium tuberculosis. Primary infection leads to active tuberculosis (A-TB) in a minority (c. 10%) of infected individuals, usually within 2 years. In the remaining cases, the immune system contains the infection, and the individual is non-infectious and symptom-free. This clinical latency can persist throughout an individual's lifetime. However, in some circumstances, the host immune response is perturbed and latent tuberculosis infection (LTBI) may develop into post-primary A-TB. This process can occur, for example, following infection with human immunodeficiency virus (HIV), malnutrition, use of steroids or other immunosuppressive medications, or because of advanced age.
Development of new diagnostic tools would improve the control of tuberculosis (TB) by improving diagnosis of A-TB and by allowing a more accurate identification of LTBI. One such test is the tuberculin skin test (TST), which has been the only widely used tool available for diagnosing TB infection. However, this test is limited by its low sensitivity (75-90%) in diagnosing A-TB, and its low specificity, caused by the fact that the purified protein derivative (PPD) used for skin testing contains >200 antigens that are shared widely among environmental mycobacteria and the Mycobacterium bovis BCG strain used for vaccination. Lastly, TST does not allow discrimination between A-TB and LTBI.
For the detection of viral pathogens, existing diagnostic test kits detect viral proteins and some measure antibody against the viral antigens. Some assays allow for a more quantitative measure of the virus in the blood stream. However, none of these tests measure the T-cell response to the pathogenic infection. Studies have demonstrated that the T-cell response (or ability to elicit a CMI response to a target antigen expressed by a pathogen) can determine if the infection can be controlled by the immune system.
Other diagnostic tests have been developed which measure the cell-mediated immune (CMI) response to a pathogenic protein. Current methods for detecting CMI responses include skin tests measuring both immediate and delayed type hypersensitivity, lymphocyte proliferation assays and measurement of cytokines produced by purified mononuclear cells cultured with antigen.
Older established methods for determining a CMI response include a proliferation assay, use the uptake of radioactive isotopes by dividing T-cells as a marker for CMI reactivity. More recently, techniques such as a single cell assay (ELISpot) have been used to detect the number of T-cells producing certain cytokines in response to the antigenic stimulation. Furthermore, most in vitro methods for detecting CMI responses involve the purification of lymphocytes from whole blood, culturing these lymphocytes with an antigen for periods from 12 hours to 6 days and then detecting T-cell reactivity to the antigen.
Some CMI assays have been developed which detect a CMI response to a peptide of a target antigen, or overlapping peptides of a target antigen. Commercially available tests to measure a CMI response to a TB target antigen include the QuantiFERON® TB Gold test (from Cellestis Ltd) and the T SPOT TB test (from Oxford Immunotec). These tests detect a CMI response to overlapping peptides of the TB target antigen. In addition to diagnosis of pathogenic infections, such as TB, CMI diagnostic tests are also useful for immune diagnosis of many diseases and disorders, including infectious and autoimmune diseases, as well as markers for immunocompetence, and for detection of T-cell responses to endogenous and exogenous antigens (i.e. vaccines).